1. The ethical concepts we use today we inherited from ancient times. They depend for their objectivity on a particular metaphysical understanding of the self that was unique to that historical period.
2. The modern metaphysical understanding of the self replaced that of the ancient world and thus removed the basis upon which ethics enjoyed any sense of objectivity.
3. The Enlightenment tried to give the fullest expression to this 'modern' understanding of the self, and attempted to provide a new rational basis for ethics on the back of this understanding, but was doomed to fail from the outset.
The moral scheme we use today originated with Aristotle and dominated the medieval period. The framework for that scheme consists of a vital distinction between man-as-he-happens-to-be and man-as-he-could-be-if-he-realized-his-essential-nature. Ethics was seen as the science that enabled humans to know how to travel from the former state to the latter state. This distinction is basically that between potentiality and its fulfilment. Ethics understood human nature as deficient, essentially, and so in need of transformation via the use of practical reason to fulfil its potential.
In order to make any sense, ethics presupposes two conceptions: (1) a conception of untutored human nature, and (2) a conception of the telos or end of that nature. Ethics is the knowledge which allows human beings to move from their present state to a new one.
The modern understanding of the self dispensed with the idea of a human telos, and thus any sense that human nature as it happens to be might be discordant with a more fulfilled state. Enlightenment philosophers (Kant, Hume, Smith) all agreed that morality’s key premises would capture some feature of human nature as it is and the rules of morality would be justified as those that a being possessing such a nature would be prepared to accept (i.e. Kant’s deontology – deriving morality from some aspect of 'how we are' (reasonable and thus autonomous) . However, since the ethical concepts they inherited and wanted to justify were originally intended to assist individuals in achieving some function (telos) rather than to fit with human nature in its untutored state, the enlightenment’s attempt to derive these ethical notions from features of present human nature was not possible. In fact, it is more likely that human nature in its untutored state would have a strong tendency to disobey the precepts of morality.
A notion of a human telos is essential to morality and ethics understood as a rationally justifiable or objective enterprise. It alone warrants deriving statements of value or obligation from statements of fact – ‘ought’ from ‘is’. We can move from the knowledge that a knife is blunt to the conclusion that it is a ‘bad’ knife because a knife is something with a telos (function/purpose). Only functional concepts are able to transform evaluative judgements into a type of factual statement.
(See Alisdair MacIntyre's beyond virtue for the whole story)
In the Ancient world, ethics derived its functional aspect, or telos, from the social roles the individual found himself in. The self was essentially characterised by its social aspect. The ethics attached to being a solder, for example, facilitated the movement of human nature as it happens to be to human nature as it should be to carry out the function of a soldier. In the modern world, on the other hand, the self is seen as antecedently individuated in that the essential element of the self apparently exists prior to any social, political or communal setting it finds itself in. Any social role the individual occupies is not constitutive of his person, on the modern view (because the essential metaphysical element of the self is characterised by freedom, autonomy and choice).
In the medieval world, ethics derived its function aspect from the Christian idea of original sin. Ethics facilitated the movement of human nature as it happens to be (in sin) to human nature as it should be, not in order to carry out some social role, but to fulfil the more universal idea of a Christian human being attaining the image of God.
While Christianity maintained a form of human telos that enabled the enterprise of objective ethics to go ahead, it could be argued that it contributed to the modern erosion of ethics via its universalist metaphysical conception of the self, as being actecedently individuated prior to social roles (the essential element of the self is the soul, in possession of the divine logos which transcends embeddedness in a particular culture or point of view)). In terms of the political-metaphysical understand of the self, Christianity represents a transitional stage that began with an Ancient, communitarian, understanding of the self as deriving its subjectivity from the social body, and ended in a modern, liberal, understanding of the self as antecedently individuated prior to all social roles and possessing all its goals, desires and outlook ‘before society’.